Tantalum metal has two crystalline phases: the low resistivity (12-20 micro-ohm-cm) alpha (bcc) (body centered cubic) phase and a higher resistivity (160-170 micro-ohm-cm) beta (tetragonal) phase. Due to the lower resistivity of the alpha phase, it is preferred for electronic applications over the beta phase.
Tantalum films are generally deposited by magnetron sputtering; however, the beta phase is usually formed for films of typical thickness (less than 3000 angstroms) which are deposited by conventional sputtering methods. This is illustrated in FIG. 1A, where beta-tantalum has been formed on a substrate.
Most of the literature is inconsistent regarding methods for reproducibly depositing a particular phase of tantalum in a predictable fashion. Some researchers have produced results which suggest that there are two main variables which determine what phase of tantalum is grown: the first variable is the substrate temperature during deposition and the second variable is the amount of gaseous contamination in the vacuum system. This has been discussed by L. Maissel and R. Glang, in "Handbook of Thin Film Technology," McGraw-Hill, page 18-12 (1970). They reported that if the substrate temperature exceeds 600.degree. C., the alpha phase is formed; also, if the base pressure of the vacuum system is high, indicating high water vapor, nitrogen, and oxygen content, the alpha phase may result.
These findings are not very useful for electronics applications because processing temperatures above 400.degree. C are typically not compatible with device fabrication. It is also difficult to maintain and control such a high substrate temperature during sputtered metal deposition. Furthermore, it is difficult to keep a controllable amount of impurities in the system.
In addition, other studies, including one by N. Schwartz and E. D. Feit, have found that they could not use these results to consistently predict which phase of tantalum would grow. See, N. Schwartz and E. D. Feit, "Impurity Effects in the Nucleation of Alpha (bcc)-Tantalum or Beta-Tantalum Films", Journal of the Electrochemical Society, Vol. 124, No. 1, pages 123-131 (January 1977). Schwartz and Feit proposed instead the existence of an "X" impurity which results in the formation of alpha tantalum. The difference between their work and that discussed by L. Maissel and R. Glang, was their hypothesis that the "X" impurity could simply be something on the substrate instead of something in the gas phase in the vacuum chamber.
Finally, G. Feinstein and R. D. Huttemann studied substrate effects on the formation of alpha tantalum. G. Feinstein and R. D. Huttemann, "Factors Controlling the Structure of Sputtered Tantalum Films", Thin Solid Films, Vol. 16, pages 129-145 (1973). They divided substrates into three groups: Group I contained substrates onto which only beta tantalum could be formed and included such substrates as 7059 glass, quartz, sapphire, and metals such as copper and nickel. Group II contained substrates onto which only alpha (bcc) tantalum could be grown. This group included substrates that had been coated with 5000 angstroms thick metal films such as gold, platinum, or tungsten. Finally, Group III consisted of substrates which normally produced alpha tantalum, but could be induced to yield beta tantalum or mixtures of alpha and beta by suitable treatment of the surface. These included substrates coated with 5000 angstroms of molybdenum, silicon nitride, or stochiometric tantalum nitride (Ta.sub.2 N). The use of such thick underlayers, as described by G. Feinstein and R. D. Huttemann makes it impractical to use this concept to produce alpha tantalum for electronics use. Since it is generally preferred to not introduce extra materials into a device structure, the use of different materials (such as tungsten or molybdenum) is impractical for electronics use.
Parisi in U.S. Pat. No.3,558,461, discloses a method for fabricating thin film tantalum based resistors by reactive sputtering of tantalum in the presence of oxygen and nitrogen, followed by anodization and thermal pre-aging of the resultant deposited film. The films were reactively sputtered in N.sub.2 and O.sub.2 containing ambient to control the TCR (Thermal Coefficient of Resistance). The resistivity ranged from 300 to 1500 micro-ohm-cm. The resulting film is referred to as "tantalum oxynitride," which is different than the alpha tantalum films that are produced by the process of this invention.
Kumagai, U. S. Pat. No. 3,847,658, discloses depositing nitrogen-doped beta-tantalum. A thin-film electrode comprising nitrogen-doped beta-tantalum is deposited upon a suitable electrically non-conductive substrate. The process produces N.sub.2 doped beta tantalum for capacitor formation, as opposed to the alpha tantalum disclosed in this invention. The films have 0.1 percent to 10 percent N.sub.2, and resistivities 10 atomic percent to 50 percent higher than that of pure beta Ta films. Furthermore, beta tantalum was grown on a Ta.sub.2 O.sub.5 underlayer, whereas, the inventors of this invention disclose the use of Ta(N) as the seed or underlayer for the formation of alpha-Ta.
Schauer in U.S. Pat. No. 3,878,079, discloses a method for producing thin films of tantalum in the alpha phase bcc (body centered cubic) lattice by heating a substrate to a temperature above 300.degree. C. and applying a high frequency discharge to a tantalum target member. The growth of alpha or beta tantalum depends on the N.sub.2 partial pressure in the sputtering gas. A nitrogen partial pressure is provided in the sputtering atmosphere, and by decreasing the nitrogen partial pressure, TaN, Ta.sub.2 N, and finally alpha tantalum were successively formed. This alpha tantalum was highly doped with foreign gases, in contrast with the invention described in this patent application which is pure alpha Ta. With further reduction in N.sub.2 partial pressure, beta Ta is formed, and with still further reduction, alpha Ta can again be formed if the substrates are heated to over 300.degree. C. The resulting film resistivity of that alpha-tantalum produced at above 300.degree. C. was 25 micro-ohm-cm. In contrast, only modest substrate heating is required in the process of this invention, and the reactive gas is only required for deposition of the seed layer, and not for the rest of the film as disclosed by Schauer. A Ta.sub.2 O.sub.5 underlayer was used by Schauer to allow for easier substrate cleaning; his underlayer is not responsible for the production of the alpha tantalum film.
Anders, U.S. Pat. No. 4,058,445, discloses a method for producing thin film tantalum capacitors having a tantalum thin film electrode mounted on a nonconducting support member. The tantalum electrode is doped with nitrogen to produce a nitrogen content in a range from the nitrogen content of beta-tantalum to that for tantalum nitride. The process of Anders' invention produces "alpha" tantalum deposited on a Ta.sub.2 O.sub.5 coated substrate by introducing N.sub.2 into the argon sputtering gas at a partial pressure of 10.sup.-5 torr. These "alpha" tantalum films have high resistivity (100 micro-ohm-cm) with presumably high N.sub.2 content.
Arcidiacono et al. in U.S. Pat. No. 4,251,326, disclose a method for fabricating thin film RC networks by forming alpha tantalum capacitor base electrodes on a substrate while simultaneously forming alpha tantalum anodization bus bars on the substrate. The alpha-tantalum film was obtained by sputtering a tantalum film on the substrate, thermally oxidizing the tantalum film to form an underlayer, and then sputtering an alpha tantalum film over the deposited Ta.sub.2 O.sub.5 underlayer. The purpose of the underlayer is to protect the substrate from attack by corrosive etchants during subsequent processing, and apparently has nothing to do with the formation of alpha Ta. This is different than the invention of the applicants where the Ta(N) seed layer or underlayer is required to produce the alpha tantalum. Alpha tantalum was deposited by Arcidiacono in a Ar-N.sub.2 atmosphere at 3-6 mT pressure. The resulting film has BCC structure and contained 10-20 percent N.sub.2. The invention of this disclosure has Ta films that have nitrogen in the seed layer only, and not throughout the bulk of the structure. Koyama et al. (U.S. Pat. No. 4,364,099) discloses tantalum thin film capacitors, which are formed on a substrate which is reactively sputtered with tantalum in a nitrogen containing atmosphere (argon gas with nitrogen gas incorporated in an amount of from 10 to 30 percent) to deposit a highly nitrogen-doped tantalum film having a nitrogen concentration of from 14 to 30 atomic percent. Subsequently a layer of an alpha-tantalum thin film to form the lower electrode is sputtered on the highly nitrogen-doped tantalum film, this second thin film has a nitrogen concentration of from 6 to 15 atomic percent. This intermediate product is then further processed to obtain a tantalum thin film capacitor. During sputtering the temperature of the substrate was maintained at 250.degree. C. The thickness of the highly nitrogen-doped tantalum film was from 100 to 200 nm (1,000 to 2,000 angstroms), and the thickness of the nitrogen doped alpha-tantalum thin layer was at least 100 nm (1,000 angstroms).